Multilayer ceramic circuit boards are well known. A green tape is made from particular glass compositions and optional ceramic powders, which are mixed with organic binders and a solvent, cast and cut to form green tapes. Wiring patterns can be screen printed onto the tape layers to carry out various functions. Vias are then punched in the green tapes and are filled with a conductor ink to connect the wiring on one green tape to wiring on another green tape. The tapes are then aligned, laminated, and fired to remove the organic materials, to sinter the metal patterns and to crystallize the glasses. This is generally carried out at temperatures below about 1000xc2x0 C., and preferably at from about 750-950xc2x0 C. The composition of the glasses determines the coefficient of thermal expansion, the dielectric constant and the compatibility of the multilayer ceramic circuit boards to various electronic components.
More recently, metal support substrates have been used to support the green tapes. The support boards lend strength to the glass layers, and, since the green tape layers can be mounted on both sides of the metal board, and can be adhered to the metal board with suitable bonding glasses, they permit increased complexity and density of circuits and devices. In addition, passive and active components, such as resistors, inductors, capacitors and the like, can be incorporated into the circuit boards for additional functionality. Thus this system, known as low temperature cofired ceramic-metal support boards, or LTCC-M, has proven to be a means for high integration of various devices and circuitry in a single package. They can also be tailored to be compatible with devices including silicon-based devices, indium phosphide-based devices and gallium arsenide-based devices, for example, by proper choice of the metal for the support board and of the glasses in the green tapes.
The ceramic layers of the LTCC-M structure must be matched to the thermal coefficient of expansion of the metal support board. Glass ceramic compositions are known that match the thermal expansion properties of various metal or metal matrix composites. These compositions are disclosed for example in U.S. Pat. No. 5,625,808 to Tormey et al; U.S. Pat. No. 6,017,642 to Kumar et al; U.S. Pat. No. 5,256,469 to Cherukuri et al; and U.S. Pat. No. 5,565,262 to Azzaro et al. U.S. Pat. No. 5,581,876 to Prabhu et al disclose bonding glasses for adhering ceramic layers to metal support substrates. These references are incorporated herein by reference.
It would be highly desirable to be able to provide an integrated package for all required components on a single metal substrate, to provide adequate and low cost temperature control, and to provide a means for hermetically sealing the integrated package.
We have found that the LTCC-M system has an additional advantage for integrated package components that run hot. The metal support board can also act as a heat sink, directly or indirectly, for devices such as semiconductor lasers, or for devices that use very dense circuitry.
The conductive metal support substrate provides excellent heat sinking. Thus components that are hot can either be directly mounted onto the metal support board, or can be mounted to conductive vias in a multilayer ceramic circuit board that lead to the metal support board. For more complex integration, the LTCC-M technology can be used to provide additional heat sinking by connecting conventional heat sinks, or thermoelectric coolers, to, or through, the support substrate. The temperature of semiconductor lasers during operation for example must be closely controlled because the wavelength of the emitted light depends on the temperature of the device and its environment.
Another type of device wherein good temperature control is required is for thermal management of flip chip packaging. Densely packed microcircuitry, and devices such as amplifiers, oscillators and the like which generate large amounts of heat, can also use LTCC-M techniques advantageously. The chip prepare for flip chip packaging can be mounted bump side up within a cavity in the ceramic layer and bump bonded to a flex circuit that connects the input/output bumps to metal traces on the top layer of ceramic. Placing the chip on a metal support board provides the cooling required for high integration chips.